Helen Duncan was born in Callender, Scotland, on the 25 November 1897. From an early age she is said to have displayed the 'gift' of medium with the spirit world. A prominent feature of her sittings was her ability to emit 'ectoplasm' from her mouth during her trances - a stringy white substance that is supposed to give form to spirits and allow them to communicate.

She made a living by conducting séances throughout Britain, during which the spirits of the dead were alleged to have appeared, talking to and even touching their relatives.

Duncan was accepted as a minister to a sizeable network of spiritualist churches and private homes, but her work was not without controversy. In 1931 she was denounced as a fraud by both the Morning Post newspaper and an organisation called the London Psychic Laboratory, which had examined her. She was also prosecuted at Edinburgh Sheriffs Court in 1933 for affray and being a 'fraudulent medium', for which she was sentenced to a fine of £10 or a month's imprisonment.

During World War Two, Duncan lived in Portsmouth, the home of the Royal Navy. In 1941, the spirit of a sailor reportedly appeared at one of her seancés announcing that he had just gone down on a vessel called the Barham. HMS 'Barham' was not officially declared lost until several months later, its sinking having been kept secret to mislead the enemy and protect morale.

Unsurprisingly, Duncan's activities attracted the attention of the authorities and on 19 January 1944, one of her séances was interrupted by a police raid during which she and three members of her audience were arrested.

Duncan was remanded in custody by Portsmouth magistrates. She was originally charged under section 4 of the Vagrancy Act (1824), under which most charges relating to fortune-telling, astrology and spiritualism were prosecuted by magistrates in the 20th century. This was considered a relatively petty charge and usually resulted in a fine if proved. She was eventually tried by jury at the Old Bailey for contravening section 4 of the Witchcraft Act of 1735, which carried the heavier potential penalty of a prison sentence.

In particular, the medium and her three sitters were accused of pretending 'to exercise or use human conjuration that through the agency of Helen Duncan spirits of deceased persons should appear to be present'. Duncan was also charged with offences under the Larceny Act for taking money 'by falsely pretending that she was in a position to bring about the appearances of the spirits of deceased persons'.

The trial caused a media sensation and was extensively covered in the newspapers, many of which revelled in printing cartoons of witches on broomsticks. At one stage, the defence announced that Duncan was prepared to demonstrate her abilities in the witness box. This amounted to conducting a séance in the court while in a state of trance and the offer was refused.

Duncan was found guilty as charged under the Witchcraft Act and sentenced to nine months in Holloway Prison, London, but she was cleared of the other offences. She was the last person in Britain to be jailed under the act, which was repealed in 1951 and replaced with the Fraudulent Mediums Act following a campaign by spiritualist and member of parliament Thomas Brooks.

There are two common misconceptions about Duncan's conviction. The first is that she was the last person in Britain to be convicted of being a witch. In fact, the Witchcraft Act was originally formulated to eradicate the belief in witches and its introduction meant that from 1735 onwards an individual could no longer be tried as a witch in England or Scotland. However, they could be fined or imprisoned for purporting to have the powers of a witch.

The second misconception is that she was the last person to be convicted under the Witchcraft Act. Again this is incorrect. Records show that the last person to be convicted under the Witchcraft Act was Jane Rebecca Yorke in late 1944. Due to her age (she was in her seventies) she received a comparatively lenient sentence and was fined.

Additionally, it has often been suggested that the reason for Duncan's imprisonment was the authorities' fear that details of the imminent D-Day landings might be revealed, and given the revelation about the Barham it is clear to see why the medium might be considered a potential risk. Nonetheless, then prime minister Winston Churchill wrote to the home secretary branding the charge 'obsolete tomfoolery'.

Helen Duncan was released from prison on the 22 September 1944 and seems to have avoided further trouble until November 1956, when the police raided a private séance in Nottingham on suspicion of fraudulent activity. No charges were brought and shortly afterwards, on 6 December in the same year, the woman who is sometimes remembered as the 'last witch' died.

A campaign by her descendents to clear her name continues to this day.